In subjects with competent lips, the lips are lightly closed during chewing food. However, in some patients with incompetent lips, the lips are not always closed during masticatory function. Chewing with the lips closed is an involuntary act in both subjects with incompetent lips and competent lips. In contrast, chewing with the lip closed is not a natural act in subjects with incompetent lips. The purpose of this study was to investigate the relationship between activities of lower lip and masticatory muscles.Results(1)The amplitudes of EMG activity of lower lip during chewing with lip in contact and apart during masseter active and non-active (tonic) contraction phases were significantly higher in the incompetent lip group than in competent lip group.(2)The duration of EMG activity during tonic masseter activity with lip contact was shorter in incompetent lip group than in competent lip group.Our results suggested that failure of lip sealing is probably due to facial morphological disharmony and that such failure also affects the masticatory function.